To test for a role for MyoD in muscle regeneration, we interbred MyoD mutant mice with mdx mice. The mdx mice lack dystrophin, but unlike humans do not develop extensive dystrophic damage. By contrast, mdx:MyoD-/-mice display severe dystrophic changes and cardiomyopathy that lead to premature death around one year of age. We propose to further characterize skeletal and cardiac muscle of mdx:MyoD-/- mice by extensive morphometric, immunohistological, physiological, and molecular analyses. Satellite cells are a distinct lineage of myogenic cells that arise late in development. To investigate the ontogeny of satellite cells, chimeric mice will be generated using Myf-5-/-:MyoD-/- embryonic stem (ES) cells constitutively expressing lacZ. To investigate the role of the Myf-5 in satellite cell self-renewal and activation, we will investigate muscle regeneration in mice carrying MyoD or myogenin knocked into the Myf-5 allele. In addition, a Cre-inducible loss-of function mutation will be knocked into the endogenous Myf-5 allele and Cre-recombinase will be expressed from the endogenous MyoD gene, or by infection with Cre-expressing adenovirus. The ETS-domain transcription factor PEA3 is rapidly induced following muscle damage and PEA3 expression stimulates myoblast differentiation and is positively correlated with metastatic potential. We propose to characterize muscle regeneration and myoblast differentiation in PEA3-/- mice that we have generated. Many lines of evidence support the assertion that cell cycle control and myogenic factor activity is coupled via a mechanism that regulates the switch from proliferation to differentiation. We have generated null mutations in members of the retinoblastoma-family of cell cycle control genes. We will analyze muscle regeneration in vivo and myogenic cell growth and differentiation in vitro to elucidate the role of cell cycle control in myogenic stem cell function. Understanding how regulatory genes control the growth and repair of skeletal muscle is highly relevant to understanding the regenerative processes that occur in patients with various muscular dystrophy's. We believe that our proposed studies will provide novel insights into the biology of muscle regeneration. Potentially, such insights may lead to new modalities of therapeutic intervention.